Method for coating a sliding element and sliding element, in particular a piston ring

ABSTRACT

The invention relates to a method wherein nanoparticle are first produced and then infused in the coating during the coating process by means of a PVD and/or CVD method. A sliding element comprises a coating formed by means of a PVD and/or CVD method comprising separately produced nanoparticles.

TECHNICAL DOMAIN

The invention relates to a method for coating a sliding element and a sliding element, in particular a piston ring. It is a requirement for sliding elements, such as piston rings, that they only ever bring about small friction losses. For example, with piston rings acting as sliding elements in internal combustion engines, an increase in friction has a direct effect upon fuel consumption. Furthermore, oil consumption is affected by the condition of the piston rings. In particular, with regard to this, the so-called burn mark strength and outbreak strength, which must be particularly high in order to permanently realise the required friction values, are to be observed.

PRIOR ART

As previously used items piston rings are known which are coated by means of PVD methods on a hard material base, in particular chromium nitride. Furthermore, the electrochemical deposition of chromium layers associated with the incorporation of A1203 or diamond particles, the size of which comes within the micrometre range, is known.

A DLC (diamond-like carbon) coating system, that can include tungsten carbide depositions in nanocrystalline form, which are produced during the separation process and are up to 10 nm in size, is revealed by WO 2007/079834 A1.

Finally, DE 199 58 473 A1 relates to a method for producing composite layers with a plasma beam source, wherein nanocrystalline particles can be embedded, and that can be combined with known, separately controllable CVD or PVD methods.

DESCRIPTION OF THE INVENTION

The object forming the basis of the invention is to make available a method for coating a sliding element and a corresponding sliding element with which the required friction and wear and tear properties can be realised over the required life span.

This object is achieved by means of the method described in claim 1.

Therefore, the invention proposes a method for coating, comprising at least one layer and formed on at least one outer surface, a sliding element, in particular a piston ring, wherein nanoparticles are initially produced, and then infused into the coating during the coating process. In other words, the nanoparticles are not produced in situ, i.e. during the coating process, but they are produced separately, to a certain extent ex situ, and incorporated into the coating during the coating process. The mechanism which can be used in this way and which leads to improved mechanical properties, such as fatigue strength, burn mark strength, outbreak strength, breaking strength and elongation at rupture, functions as follows according to the current state of knowledge. It is also noted that the invention is not restricted to this. The incorporation of the described particles gives rise to local crystal lattice deformations which lead to the aforementioned, improved mechanical properties. Furthermore, an improvement of the wear and tear characteristics due to the exceptionally high grain limit density and increased elasticity and less friction are achieved.

The advantages of the infused nanoparticles can also be made use of in the dispersion or precipitation hardening to be implemented. That is to say, the displacements produced when stressed or already existing cannot be worked or “cut” through by the particles or the depositions, but bulge out to a certain extent between the particles. In this way, displacement rings are formed which must be bypassed by the displacements. With this bypassing, higher energy is required than when the latter are “cut through” by the particles or depositions. The loading capacity is thus increased. Furthermore, the invention advantageously further makes use of the effect that the yield stress for the migration of the displacements increases as the particle spacing decreases and the particle size decreases. The material strength increases due to this. This effect can be obtained particularly well with nanoparticles. Furthermore, it has been shown within the scope of the invention that upon the basis of their high defect density on the surface, the latter can be infused and incorporated practically independently of the material to be reinforced during the coating process. In this way, the desired depositions, which can be incoherent, partially coherent or coherent, and have the effects described above with regard to the mechanical properties, can advantageously be formed. The production of the nanoparticles ex situ advantageously further guarantees that the chemical and crystallographic structure of the nanoparticles can be controlled. Furthermore, by means of this control, when producing the nanoparticles it can be guaranteed that the latter can be infused into the layer hereby growing during the coating process in the desired manner.

The coating as such is advantageously implemented by means of tried and tested PVD (physical vapour deposition) and/or CVD (chemical vapour deposition) coating processes.

Advantageous further developments of the method according to the invention are described in the further claims.

For the base material or the matrix of the coating a material that contains nitrides, in particular metal (oxy)nitrides, and in particular Cr(O)N, AlN or TiN, has proven to be particularly advantageous.

In initial trials it transpired that a volume portion of the nanoparticles of 20% or less leads to good properties.

Furthermore, one was able to have good experiences with nanoparticles which have a particle size (diameter) of 1 to 100 nm, preferably 5 to 75 and in particular 5 to 50 nm.

For the nanoparticles compounds from the group of oxides, carbides and/or silicides with the composition Me_(x)O_(y), Me_(x)C_(y) or Me_(x)Si_(y) are preferred. The metal here can be chromium, titanium, tantalum, silicon, indium, tin, aluminium, tungsten, vanadium or molybdenum, and/or x can be 1 to 3 and/or y can be 1 to 3.

With regard to the layer thickness, particularly good properties have been determined with a coating thickness of max. 100 μm, and preferably in the range of 5 to 50 μm.

Even though the coating according to the invention can be used in many different ways, due to the proven properties it is currently preferred if the base material, i.e. the coating material of the sliding element to be coated according to the invention, is cast iron or steel.

The aforementioned object is achieved, furthermore, by the sliding element described in claim 9, wherein this is particularly a piston ring. The preferred embodiments of the sliding element according to the invention correspond to those of the method according to the invention for producing the latter. This applies in the same way to the advantages that occur, which lie in particular in a permanent sliding element permanently having the required friction values and wear and tear properties.

For the preferred case of a piston ring, it is mentioned that as sliding surfaces, one or more faces, i.e. the upper and/or the lower side and/or the contact surface, i.e. the outer cylinder surface of the piston ring, can be coated. The contact surface can be coated more thickly with the coating according to the invention with incorporated, separately produced nanoparticles than at least one of the faces. The cross-over between the contact surface and at least one face can be rounded on the coating, in the same way as this cross-over on the base material of the piston ring can be rounded. The coating of both faces can be of the same thickness. In particular applications also only the contact surface can be coated. 

1. A method for coating, comprising at least one layer and formed on at least one outer surface, a sliding element, wherein nanoparticles are initially produced, and then infused into the coating during the coating process, which is implemented by means of a PVD and/or a CVD method, the coating being formed containing a metal oxynitride.
 2. The method according to claim 1, wherein the coating is formed containing a metal nitride.
 3. The method according to claim 1 wherein the coating is formed containing CrON.
 4. The method according to claim 1, wherein the nanoparticles comprise up to 20 volume % of the coating.
 5. The method according to claim 1, wherein the coating is formed such that the nanoparticles are 1 to 100 nm in size.
 6. The method according to claim 1, wherein the coating is formed such that the nanoparticles are chosen from the group of oxides, carbides and/or silicides, and one or more of the compounds comprise Me_(x)O_(y), Me_(x)C_(y) and Me_(x)Si_(y) with Me: Cr, Ti, Ta, Si, In, Sn, Al, W, V, Mo and/or x=1 to 3 and/or Y=1 to
 3. 7. The method according to claim 1, wherein the coating is formed with a total thickness of up to about 100 μm.
 8. The method according to claim 1, wherein the coating is formed over cast iron or steel as the base material of the sliding element.
 9. A sliding element with a coating comprising at least one layer, formed by means of a PVD and/or a CVD method, on at least one outer surface, which has separately produced nanoparticles, wherein the coating contains a metal oxynitride.
 10. The sliding element according to claim 9, wherein the coating contains a metal nitride.
 11. The sliding element according to claim 9, wherein the coating contains CrON.
 12. The sliding element according to claim 9, wherein the nanoparticles comprise of up to 20 volume % of the coating.
 13. The sliding element according to claim 9, wherein the nanoparticles are 1 to 100 nm in size.
 14. The sliding element according to claim 9, wherein the nanoparticles are chosen from the group of oxides, carbides and/or silicides, and one or more of the compounds comprise Me_(x)O_(y), Me_(x)C_(y) and Me_(x)Si_(y) with Me: Cr, Ti, Ta, Si, In, Sn, Al, W, V, Mo and/or x=1 to 3 and/or Y=1 to
 3. 15. The sliding element according to claim 9, wherein the whole thickness of the coating is up to about 100 μm.
 16. The sliding element according to claim 9, wherein the base material of the sliding element comprises cast iron or steel.
 17. The method of claim 1, wherein the sliding element is a piston ring.
 18. The method of claim 2, wherein the metal nitride is selected from at least one of CrN, AlN or TiN.
 19. The method of claim 5, wherein the nanoparticles are 5 to 75 nm in size.
 20. The method of claim 5, wherein the nanoparticles are 5 to 50 nm in size.
 21. The method of claim 7, wherein the total thickness is 5 to 50 μm.
 22. The sliding element of claim 9, comprising a piston ring.
 23. The sliding element of claim 10, wherein the metal nitride comprises at least one of CrN, AlN or TiN.
 24. The sliding element of claim 13, wherein the nanoparticles are 5 to 75 nm in size.
 25. The sliding element of claim 13, wherein the nanoparticles are 5 to 50 nm in size.
 26. The sliding element of claim 15, wherein the total thickness is 5 to 50 μm. 